The performance of Global Navigation Satellite Systems (GNSS), such as the Global Positioning System (GPS) and other satellite based navigational systems, may be enhanced by inertial navigation systems (INSs), which utilize inertial measurement units such as gyroscopes and accelerometers to measure changes in direction and acceleration. See FIG. 1. Other inertial measurements that may be used are car odometer readings, wheel ticks from car wheels, and angle of a car steering wheel. INSs may be integrated into a very wide range of satellite based navigation devices—e.g. personal navigation devices (PNDs), other hand held devices such as cell phones, and vehicle-based navigation devices.
Many devices with integrated GNSS may only have accelerometers, in which case the platform orientation is difficult to obtain. For example, accelerometers are integrated into cell phones with cameras, to assist in image stabilization. In the absence of platform orientation with respect to natural reference frames the data from accelerometers is not suitable for integration into platform velocity and further into platform position. In such cases a full INS integrated algorithm cannot be implemented.
Furthermore, these accelerometers are often low cost and are only sufficient to allow dynamic detection, direction pointing or map orientation. In many applications and for many receiver platforms low cost accelerometers do not allow for INS type integration with a GNSS system.
There is a desire to use data from these integrated accelerometers, without requiring INS integration, to improve GNSS performance in aspects of satellite navigation and tracking.
For devices where full 6 degrees of freedom sensors are available, the sensors still need to be fully and accurately aligned and calibrated before a reliable INS integrated algorithm can proceed. Further, the accuracy of an INS, and how long it remains accurate, depends on the quality of the sensors and how well they are calibrated. Yet further, in most cases the calibration of the sensors needs to be updated periodically as the sensor parameters (like sensor bias) change with time, temperature of the system, etc. Furthermore, calibration of the INS sensors is required each time before starting the process of inertial navigation. The calibration of an accelerometer triad, for example, might include determining: the orientation of the triad relative to the gravity vector; the biases of the individual accelerometers; and may be even scaling factors for the individual accelerometers. Until the calibration process is complete, inertial navigation will not be available. In prior art systems, before the accurate alignment and calibration is complete, the sensor data is not used to provide navigation assistance.
There is a desire for methods and systems for GNSS enhancement that are available without having to wait for alignment and calibration of sensors.